Remove multimap usage from memoizeDeclsByName

[asl]

Use used std::unordered_multimap to store declarations within a namespace having the same name. It turned out it was a bit of overkill:

• In the vast majority of cases namespace only has unique declarations
• There was no multimap in Abseil, so we were unable to use these more efficient maps
• Profiling shows some elevated malloc traffic around these multimaps in downstream applications

This PR switches to:

• Abseil flat_hash_map to store declarations given name
• Use inlined vector to store up to 2 declarations inline within a map (so no additional memory allocation is required in the vast majority of cases)

Few improvements along the way. No functionality change

[asl]

Impact on one downstream application:

Command	Mean [s]	Min [s]	Max [s]	Relative
baseline	61.816 ± 0.596	61.160	64.674	1.02 ± 0.01
this PR	61.388 ± 0.950	60.019	62.715	1.00

yielding small (2%) but observable speedup.